1. Field of the Invention
The present invention relates to a fixing unit for fixing a toner image onto a recording sheet such as paper by applying heat to the recording sheet which passes through two rollers contacting each other, and relates to an image forming apparatus having such a fixing unit. The present invention is applied to various types of image forming apparatuses, e.g., a copier, a printer, a facsimile machine, and a combination machine thereof.
2. Description of the Related Art
Image forming apparatuses such as a printer and a copier, and multi-functional image forming apparatuses called combination machines or Multi Function. Peripherals (MFP) have conventionally been used. Such an image forming apparatus is provided with a fixing unit which applies heat to loose toner powder coated on a recording sheet such as paper through electrophotography to fix the same on the recording sheet.
The fixing unit includes a heated roller containing a heater therein along the axial direction of the heated roller, and a pressure roller provided to contact the heated roller for rotation. The fixing unit is so structured that paper or the like passes through a nip between the two rollers contacting each other.
There is proposed a device which has, as a heater (heat source) for heating a heated roller, two heaters differing from each other in distribution of the amount of heat generated in the length direction thereof. The two heaters are selectively used in the device (Japanese Laid-open Patent Publication No. 2011-118261).
To be specific, according to the device described in Japanese Laid-open Patent Publication No. 2011-118261, the two heaters of a main heater and a subordinate heater are provided. A switch temperature is stored for drive of the two rollers, and the switch temperature is compared with the temperature of a fixing roller, and depending on the comparison result, any one of the two rollers is selected and driven. Further, the method for switching between the two heaters is changed depending on whether the roller rotates or not. This reduces the maximum consumption power and falls the temperature within a predetermined range.
Meanwhile, in a fixing unit provided with a heated roller containing a single heater therein, heat escapes from the ends, in the axial direction, of the heated roller even in a standby mode in which no paper is conveyed. Thus, a tendency is seen in which temperature in the ends of the heated roller drops, in particular, in the standby mode. For this reason, immediately after the mode is changed from the standby mode to the print mode and paper feed starts, a fixing failure probably occurs due to the low temperature in the ends of the heated roller. In order to cope with this, it is necessary to, for example, set light distribution to (amount of heat generated in) the ends of the heater at a high value so as to prevent the temperature in the ends of the heated roller from dropping.
When a plurality of pieces of paper is continuously delivered in the print mode, heat of the heated roller is transferred from a part thereof corresponding to the paper width to the paper passing through the nip. As a result, the temperature of the heated roller drops. To cope with this, control (heater control) is made in such a manner that a heated time (ON time) of the heater is extended to prevent the temperature of the heated roller from dropping. In such a case, the temperature in the ends of the heated roller corresponding to outer regions beyond the paper width does not drop because no heat is transferred to the paper, and further, heat is conducted from the center of the heated roller to the outer regions. Further, the temperature in the ends of the heated roller is increased because the heated time thereof is increased.
Such a problem occurs not only in a fixing unit having a single heater, but probably in a fixing unit having a plurality of heaters corresponding to paper size as disclosed in Japanese Laid-open Patent Publication No. 2011-118261.
To be specific, in conventional fixing units, when a plurality of pieces of paper is continuously delivered in the print mode, the temperature in the ends of the roller rises greatly. In the case where silicone rubber is used in the fixing unit, Ultra-Fine Particles (UFP) having a size equal to or smaller than 100 nm tend to be generated. It is desirable to minimize the amount of generation of the UFP.